Im wondering how could I prepare 2 different resolutions of assets for different screen sizes and use it without much of a hassle from construct because:there is a lot of lower end devices that need to use 320x480 in order to have any performance but at the same time another big chunk of devices would look much better with 640x960 (for example kindle fire hd with its incredibly high res of 1920x1080 or ipad3).

Do you have an example to show the difference in performance?C2's scale future works pretty well in fitting it's games on different screen sizes.The only performance issue I can imagine is in the amount of ram needed for the sprites. Adding more sprites would make that even worse.You could choose to make two versions of your game, a normal and a HD one, most code could just be copied, all you'd have to do is change the graphics. But as I said before, if you just take into account how much ram you are using with your graphics, you could just use C2's scale option.

I told my dentist I had trouble with my teeth and asked her to fix it without looking in my mouth..

I actually dont have example right now because im wrapping up my first game, after Ive seen terrible performance on iphone4 (around 30 fps with constant lag) Ive switched to non retina assets and upscale it, but turns out most of my issues existed because I use very big scroll-tiled image wchich isnt really that cool for canvas rendering :)

I would say case closed as its very easy to swap all art assets after game is done. Probably one day of work.